The present invention is related generally to coating pigment-containing materials on substrates. More specifically, the present invention is related to the high speed coating of pigment-containing liquid coating materials on webs or liners. Even more specifically, the present invention is related to high speed coating of such pigmented coating materials so as to avoid visible pigment separation. The present invention finds one use in coating liquid organosol pigment materials onto webs or liners.
Films, tapes, and other substrates have long been coated with colored pigment-containing liquid coating materials. Thin, flexible substrates such as, for example, films, webs or liners have been coated using roll coating techniques which often include rolls for feeding, coating, and taking up the finished, coated product. Roll coating methods have included applying a coating material using a rotating applicator roll that transfers the coating material from a feed pan to a moving substrate. The substrate is usually positioned over a feed roll so that when the feed roll is rotated the substrate is moved longitudinally past the applicator roll. The feed and applicator rolls are positioned so that the coating material is at least partially transferred from the applicator roll to form a coated layer on the moving substrate. Die coating techniques have also been used to apply such liquid coating materials to a substrate. In a typical die coating process, the coating material is applied to the surface of the moving substrate through a die.
Colored coating materials typically include pigments that are generally dispersed or suspended evenly throughout a liquid so as to provide an even color to the coating material. When the pigmented coating is applied to a substrate, an even color appearance is preferable for most applications, and is required by many others.
Colored coatings made using conventional methods have been known to exhibit an uneven color appearance during some production runs. The uneven color appearance can take the form of streaks, swirls or other shapes having a color intensity that is different than that of the surrounding coated layer. The streaking is typically oriented in the machine or longitudinal direction and visible with normal unaided human eyesight. The uneven color appearance has been found on the interface surface or underside of the coated layer (i.e., the surface of the coated layer that is formed on the moving substrate) and, therefore, can go unnoticed until after the coated layer has been formed and cured. In addition, the appearance of such streaking problems increase with the use of higher line speeds (i.e., the speed at which the substrate is moving). This problem has, thus, resulted in the use of lower than desired production rates. Such uneven color problems have been experienced for decades without the cause being identified.
Accordingly, there is a need for a solution to this uneven color problem.
The present invention provides a solution to the problems associated with the uneven color appearance, at least in part, by providing a method of high speed coating a pigment-containing liquid coating material onto a moving substrate so as to avoid visible pigment separation in the coating material in its as coated state, at least on the surface of the coated layer to be viewed.
It has been found that the uneven color appearance experienced in prior art is the result of pigment separation that is visible, with normal unaided human eyesight, on a surface of the coated layer, typically its interface surface. As used herein, pigment separation is a non-uniform distribution or localized concentration of the pigment used in the liquid coating material. It has also been found that the use of a substantially straight wetting line when applying the coating material onto a moving substrate can significantly reduce, if not completely eliminate, this pigment separation and, thereby, the uneven color appearance problem. It has further been found that organosol coatings in general, and especially organosol coatings containing metallic or other flake-shaped pigments, are particularly prone to such pigment separation problems. Such pigment separation problems can be seen in coatings where pigments of different sizes and/or types, such as small particles and larger flakes, are both present in the coating material. Such pigment separation problems can also be problematic for translucent films designed to be displayed with backlighting when pigments of different sizes and/or types, such as smaller transparent pigments and larger opaque or flake pigments, are both present in the coating material. It has also been found that pigment separation problems are most noticeable on the interface surface or underside of the coated layer (i.e., the surface of the coated layer that once contacted the moving substrate). When a transparent coating (e.g., a convention clear coat) is positioned over the underside of the coated layer, pigment separation can be seen through the transparent coating.
In one aspect of the present invention, a method is provided that comprises providing a first substrate (e.g., a film, web or liner) having a coating surface to be coated and providing a pigment-containing liquid coating material. The coating material is applied to the surface of the first substrate along a substantially straight wetting line to form a coated layer having an interface surface in contact with the coating surface of the first substrate. The coating material is applied while the first substrate is moving at a high line speed of at least about 50 ft./min. (15.24 m/min.). The pigment-containing coating material is of the type that, without the use of a substantially straight wetting line, will exhibit visible (i.e., visible with normal unaided human eyesight) pigment separation on its interface surface when the coating material is coated onto the coating surface of the first substrate at the high line speed. For the purposes of the present invention, a wetting line is substantially straight when visible pigment separation does not occur at the high line speed being used. With regard to the present invention, a high line speed is when the first substrate is coated while moving at a rate of at least about 50 ft./min. (15.24 m/min.). It can be desirable for the wetting line to be sufficiently straight to permit line speeds of at least about 60 ft./min. (18.29 m/min.), without producing visible pigment separation on the surface of the coated material. It can also be desirable for the wetting line to be sufficiently straight to permit line speeds of at least about 70 ft./min. (21.34 m/min.), 80 ft./min. (24.38 m/min.), 90 ft./min. (27.43 m/min.) or 100 ft./min. (30.48 m/min.). It can further be desirable for the wetting line to be sufficiently straight to permit line speeds of greater than 100 ft./min. (30.48 m/min.).
This method can include removing the coated layer from the coating surface of the first substrate to expose the interface surface of the coated layer. A second substrate can then be provided and adhered to the adhering surface of the coated layer. The adhering surface of the coated layer is opposite its interface surface.
In another aspect of the present invention a method of making an article is provided. The method comprises making a coated layer by high speed coating a pigment-containing liquid coating material onto a substrate as described above. An article is then made using the coated layer. The article being so made can be a color coated article, where the method further comprises removing the coated layer from the coating surface of the first substrate to expose the interface surface of the coated layer. The article is then made, at least in part, by adhering the coated layer (e.g., with a pressure sensitive adhesive) to another substrate, with the interface surface exposed. This other substrate can form part (e.g., body part, trim, etc.) of a vehicle such as, for example, an automobile, aircraft or watercraft. The other substrate can also be an intermediate substrate, like a release liner, or a separate film or other part of an article made using the coated layer.
One method for applying coating material along a substantially straight wetting includes reverse roll application of the coating material. Reverse roll application can additionally provide quick changeover relative to die or slot feed coating methods. Reverse roll application includes rotating an applicator roll in a direction opposite to the direction of substrate movement at the point of contact between the liquid coating material and the applicator roll. This may include looping the web substrate backside surface around a feed roller rotating in a direction opposite to the direction of the applicator roll. The reverse roll application may be followed by wiping excess coating material from the coated substrate with a metering device or knife. The metering knife can be a notch bar. The metering function can also be provided by another roll, for example, a reverse rotating roll.
Reverse roll application can include providing a first feed roll rotating in a first direction and a second applicator roll rotating in a second direction opposite the first roll rotation direction, with the first and second rolls forming a roll gap therebetween. The two rolls reach a point of minimum clearance at the roll gap. The substrate to be coated, or first substrate, such as a web or liner, can be passed around the first roll under tension, and through the roll gap, with the first roll rotating at a speed and direction matching the speed and direction of the first substrate. The second roll may have liquid coating material deposited on the roll using many devices, including flow bars. The second roll may at least partially be disposed in a pan of liquid coating material, with the second roll rotation carrying the coating material into the roll gap and into contact with the first substrate coating surface moving in an the opposite direction to the second roll carrying the coating material. The coating material contacts the web or liner coating surface along a substantially straight wetting line near the roll gap. The straight wetting line reduces pigment agglomeration and resulting uneven color appearance in the finished product. The coated first substrate may have excess coating material wiped or metered by passing under and near a notch bar, knife, or a third rotating roll, with the web being disposed at a controlled notch bar gap distance from the notch bar.
Another method for providing a substantially straight dynamic wetting line can include die or curtain coating the first substrate with a pigment-containing coating material. The first substrate may be passed around a feed roller under tension and a die disposed near the first substrate driven by the first roller surface. Coating material may be applied through a die orifice to the first substrate surface. The die orifice may be oriented substantially parallel to the first substrate surface, and substantially orthogonal to the machine direction or direction of first substrate movement.
After applying the coating material to the first substrate along a substantially straight wetting line, the coating layer can be separated or stripped from the first substrate. In one method, a second substrate is applied and adhered to the coated first substrate. The second substrate and adhered coating layer may be removed together, peeling the coating layer away from the first substrate, exposing the side of the coating layer that previously adhered to the first substrate. The exposed coating layer surface has a substantially even color appearance due to the application of the coating material to the first substrate along the substantially straight wetting line.
In one use of the present invention, a paper or polyester web or liner is used as a first substrate. A metallic organosol is applied along a straight wetting line to the web. A second substrate including a pressure-sensitive adhesive and release liner is applied and adhered to the metallic organosol coated web. The release liner and pressure-sensitive adhesive are peeled off of the web, taking the adhered metallic organosol layer off with the release liner and pressure-sensitive adhesive. The underside of the metallic organosol layer, previously adhered to the web, forms the coated surface of the second substrate. The exposed surface has an even color appearance. In particular, the exposed surface is substantially free of longitudinal streaks in the machine direction.